Recently, in electrophotographic image forming apparatuses, heating roller-type fixing apparatuses have come into practical use with which stable fixing is possible while having a small size. This sort of heating roller-type fixing apparatus is configured such the it is provided with a heating roller that transports, while heating, recording paper to which a toner image constituted from powdered developer (toner) has been transferred, and a pressure roller that transports the recording paper while pressing it against this heating roller, and by allowing the recording material (paper) to pass at a fixing point, which is a pressing portion (nip portion) between the heating roller and the pressure roller, the toner image on the recording material is melted and pressed (fixed).
In a heating roller-type fixing apparatus, a heating system using a heat lamp is adopted. In an ordinary heat lamp heating system, a halogen lamp, which is a heat source, is disposed inside a heat generating structure such as a heating roller, and heating is performed uniformly from inside that heat generating structure.
In a heating roller-type fixing apparatus, conventionally, an aluminum member with a large heat capacity, whose walls have been made somewhat thick, is used as the heating roller, but with this sort of heating roller, there are the problems that not only does the start time until reaching a predetermined temperature (for example, 180° C.) necessary for fixing become long, but power consumption also becomes high, and so a way of dealing with those problems is sought.
Accordingly, tests have recently been performed in which the above problems are dealt with by attempting to make the wall of the heating roller thinner and reduce the heat capacity of the fixing apparatus, in order to shorten the start time of the fixing apparatus and aim for an energy conservation effect. Further, there is vigorous development of fixing apparatuses in which, by causing the heating roller or a film member itself to generate heat with electromagnetic induction heating instead of using a conventional heat lamp heating system, the start time of the fixing apparatus is further shortened, aiming for energy conservation.
With induction heating fixing systems, there may be a configuration in which an induction coil, which is a heat source, is disposed inside the heating structure of the heating roller or the like (internal heating system), and a configuration in which the induction coil is disposed outside the heating structure, i.e., facing the side that makes contact with a print face (external heating system). As the heating structure, below configurations (1) to (3) and the like are known.    (1) a heating structure in which a comparatively thick-walled metal roller is used and the entire roller is caused to generate heat    (2) a heating structure in which sliding heat generating mm of tens of μm is caused to generate heat    (3) a heating structure configured as a roller structure in which an elastic body such as an insulating sponge is provided inside a very thin metal heat generating layer, inside of which a cored bar is provided.
Of these configurations, with the configuration using a metal roller in (1), heat capacity is large in order to obtain rigidity as a roller structure, and so it is difficult to dramatically improve the startup time.
Also, with the film-sliding system in (2), the heat capacity of the heat generating layer is very small, so there is effect of shortening the startup time, but stable rotational driving is difficult, so this configuration is unsuitable for increasing speed.
On the other hand, with the roller structure system of (3), in which an elastic sponge layer is provided inside a thin metal heat generating layer, inside of which a cored bar is of metal or the like is provided, the heat generating layer is very thin and has a small heat capacity, same as the film-sliding system, and an insulating layer is also provided in this roller structure system, so that while being a roller structure, startup time can be shortened, and stable rotation is possible even at high speed due to the roller structure. Also, because the heating roller is an elastic structure, it is possible to form a wide nip, and so this configuration is suitable for a color fixing apparatus.
However, with this sort of roller structure in configuration (3), it is fundamentally necessary to externally dispose an induction coil. With a structure in which an induction coil is externally disposed, usually, the induction coil is disposed with a configuration such that it covers a semi-circumferential portion of the roller. Thus there exists a heating portion and a non-heating portion of the heating roller (partial heating), and when the heating roller is rotating, the heat generating layer gradually is introduced to a position facing the induction coil, and so the heating roller is uniformly heated. However, when heating is performed in a state in which the heating roller is not rotating, the heated region and the non-heated region become intermingled.
On the other hand, in a copier machine, from the standpoint of a user, the first-copy-out time is an important property, and as a method of realizing this property, a method is conceivable in which a standby mode is set, and in standby mode the heating roller is pre-heated. When the heating roller is pre-heated in standby mode, it is necessary to control the temperature of the heating roller using a temperature detection member. Various proposals have been made with respect to the position in which to install the temperature detection sensor (for example, see Patent Documents 1 and 2).
Patent Document 1: JP H10-104975A
Patent Document 2: JP 2002-72755A